1. Field of the Invention
The present invention relates to a photometer in which fluorescent radiation is detected by a very sensitive photomultiplier. More specifically, the present invention relates to an assembly which prevents ambient light from striking the photomultiplier tube when a compartment for receiving a sample-containing cuvette is open but which allows fluorescent light emitted by the chemical sample to strike the photomultiplier tube when the compartment is closed.
2. Description of the Prior Art
Heretofore, difficulties have been encountered in protecting from ambient light a very sensitive photomultiplier for detecting weak fluorescent light emitted by a sample being irradiated by a light source. One method of preventing damage to the photomultiplier tube is to electrically turn off the circuit to the photomultiplier tube before a first compartment for receiving a sample-containing cuvette is opened. This method has the disadvantage that the gain of the photomultiplier tube may shift when the voltage to the photomultiplier tube is turned off and turned on again. To alleviate, if not altogether obviate this problem, the present invention provides an assembly with interlocking parts which permit the weak fluorescent light to illuminate the photomultiplier tube when the first compartment is closed and which shields the photomultiplier tube from strong ambient room light when the first compartment is opened.
As will be described in greater detail hereinafter, the present invention provides a very simple and inexpensive mechanical assembly for protecting a sensitive photomultiplier tube used to detect fluorescent radiation in a photometer from incident room light.